The present invention relates to a portable engine working machine and a rotary carburetor incorporated therein.
Portable engine working machines specifically include chain saws, bush cutters, hedge trimmers, power blowers, etc. Portable engines are often equipped with carburetors.
Portable engine working machines are increasingly computerized, and an example thereof is a solenoid valve adopted for fuel control (Patent Documents 1, 2). In Patent Document 1, an engine including a solenoid valve for fuel control employs a combination of the solenoid valve and a butterfly carburetor. On the other hand, in Patent Document 2, an engine including a solenoid valve for fuel control employs a combination of the solenoid valve and a rotary carburetor.
The butterfly carburetor disclosed in Patent Document 1 has a discharge part facing an intra-carburetor air-fuel mixture passage, and fuel is sucked through this discharge part into an intake passage in the carburetor. Similarly, the rotary carburetor disclosed in Patent Document 2 has a nozzle projected into an intra-carburetor mixture passage, and fuel is sucked through this nozzle into an intake passage in the carburetor. Therefore, the fuel is supplied from the discharge part to the intake passage by utilizing a negative pressure of the intra-carburetor air-fuel mixture passage. The solenoid valve is disposed in an intra-carburetor fuel supply passage leading to the discharge part or the nozzle. It is noted that the rotary carburetor disclosed in Patent Document 2 does not include a needle inserted into a tip portion of the nozzle to control a fuel discharge amount. This is specified in paragraph [0038] of Patent Document 2.
As understood from Patent Documents 1 and 2, the rotary carburetor is employed as a carburetor incorporated in a portable engine working machine in addition to the butterfly carburetor. A basic structure of the rotary carburetor has a valve main body rotatable in a carburetor main body, a nozzle arranged coaxially with a rotation axis of the valve main body, and a needle inserted into the nozzle from a tip of the nozzle. An effective cross-sectional area of an intake passage is controlled by rotation of the valve main body. A fuel discharge amount is controlled by movement of the needle relative to the nozzle. In Patent Document 2, as described above, it is proposed that the needle is eliminated to interpose the solenoid valve instead in the intra-carburetor fuel supply passage leading to the needle.
An example of a rotary carburetor incorporated in a portable engine working machine is disclosed in Patent Document 3. In the rotary carburetor disclosed in Patent Document 3, a nozzle is stationary in a non-rotatable manner. The nozzle has a fuel discharge port at a tip thereof, and this fuel discharge port has a tapered shape in a circumferential direction. A needle is rotatable in conjunction with a valve main body and rotates around an axis together with the valve main body. The needle also has an opening vertically extending in a circumferential surface thereof. The valve main body and the needle described above are linked to a throttle lever operated by an operator for output adjustment such that the valve main body and the needle rotate around an axis.
When the operator operates the throttle lever, the valve main body and the needle rotate around an axis. This changes the effective cross-sectional area of the intake passage, i.e., a throttle valve opening degree. Additionally, the rotation of the needle relative to the stationary nozzle changes an area of an effective fuel outlet formed when the opening of the needle coincides with the fuel discharge port of the nozzle. Consequently, the rotary carburetor has the fuel discharge amount mechanically controlled together with the effective cross-sectional area of the intake passage (the throttle valve opening degree).
Patent Document 4 discloses a most popular rotary carburetor in the portable engine working machine. In the rotary carburetor of Patent Document 4, a nozzle is stationary in a non-rotatable manner. A rotatable valve main body is displaceable in the axial direction of the nozzle because of a support pin and a cam surface engaged therewith. A needle is integrated with the valve main body. The needle is displaced in the axial direction in conjunction with the axial rotation of the valve main body and the displacement in the axial direction associated therewith. The nozzle has a fuel discharge port on a circumferential surface of a tip portion thereof, and the effective opening area of the fuel discharge port is controlled by an advancing/retracting movement of the needle inserted into the tip of the nozzle. In other words, the fuel discharge amount is controlled by the relative advancing/retreating movement of the needle.
When the operator operates the throttle lever, the valve main body mechanically linked thereto rotates. An effective cross-sectional area of the intake passage in the carburetor, i.e., the throttle valve opening degree, changes according to the rotation of the valve main body. The rotation of the valve main body induces an axial displacement of the valve main body due to the cam surface. The axial displacement of the valve main body is accompanied by a relative displacement of the needle in the axial direction. On the other hand, since the nozzle is stationary, the effective opening area of the fuel discharge port of the nozzle circumferential surface varies according to the displacement of the nozzle in the axial direction.
Patent Document 5 discloses a rotary carburetor applied to a stratified scavenging engine. The stratified scavenging engine includes a scavenging passage communicating with a crank chamber and a combustion chamber, and this scavenging passage is charged with air. In a scavenging stroke, the air in the scavenging passage is first supplied to the combustion chamber, and an air-fuel mixture is then supplied from the crank chamber through the scavenging passage to the combustion chamber. The rotary carburetor disclosed in Patent Document 5 has two passages formed in a rotatable valve main body. One is a passage generating the air-fuel mixture, and the nozzle described above is arranged in this intra-carburetor air-fuel mixture passage. The other is a passage for supplying air to the scavenging passage.